Nanometer-sized ultrafine particles containing a metal element as a constituent element (for example, metal oxide nanoparticles and metal hydroxide nanoparticles) are expected to be used in the wide fields of catalysts, storage materials, light-emitting materials, fluorescent materials, secondary battery materials, electronic component materials, magnetic recording materials, polishing materials, optoelectronics, pharmaceuticals, cosmetics, and the like. It is known that materials using nanometer-sized particles often exhibit interesting characteristics attributable to the extremely small sizes. Reportedly, some of engineering, electronic, mechanical, and chemical characteristics exhibited by these materials are different from those of already-existing bulk materials. In particular, magnetic nanoparticles have attracted increasing attention, and are started to be researched actively. Notable and attractive properties among characteristics exhibited by metal element-containing nanoparticles including magnetic nanoparticles are closely related with quantum properties and magneto-optical property, and have attracted industrial and scientific attention in the wide applications. Magnetic nanoparticles are expected to find many applications such as ferrofluids, high-density recording materials, and medical diagnostic materials.
Since magnetic nanoparticles have promising applications, magnetic nanoparticles have attracted increasing attention from researchers in wide fields (NPLs 1 to 4). Magnetite (magnetic iron ore, Fe3O4) is chemically nontoxic to the human. Hence, a lot of medical applications of Fe3O4 have been proposed, and Fe3O4 is being researched for those medical applications. The proposed medical applications include, for example, an application as a carrier for drug or gene delivery, an application in hyperthermia therapy of cancer, applications in biosensors, and applications in the tissue engineering including regeneration and transplantation medicine (NPLs 5 to 8).
To achieve such medical applications, Fe3O4 is required to be sufficiently small, and be dispersed well in water or blood, without aggregation. In addition, Fe3O4 is required to evade capture by phagocytes including macrophages, i.e., to be stealthy to immunological reactions in the human body. Fe3O4 nanoparticles are synthesized by various methods (NPL 9), and surface properties of Fe3O4 nanoparticles can be altered by several approaches including the ligand exchange method (NPLs 10 to 15)
To evade the capture by phagocytes and extend the half-life in blood (NPL 16), coating iron oxide nanoparticles with various polymers such as polyethylene glycol has been attempted (NPLs 17 to 20). However, these methods are each practically difficult to apply, and have a problem associated with the stability of a dispersion during transfer between solvents (NPL 21).
In addition, nanoparticles greater than 200 nm are reported to be captured by phagocytes in the spleen (NPL 22).
Note that the group of the present inventors has developed a technology using a hydrothermal synthesis system under high-temperature and high-pressure water such as subcritical water or supercritical water as an approach for synthesizing metal oxide nanoparticles or metal hydroxide nanoparticles (PTLs 1 to 5).